One of the hottest healthy snack food companies is suddenly under fire

The Food and Drug Administration is cracking down on the maker of Kind snack bars for marketing its products as healthy.

In a warning letter addressed to Kind, the FDA said at least four types of Kind bars are in violation of "healthy" labeling rules, Bloomberg reports.

The bars in question include the Almond & Apricot, Almond & Coconut, Peanut Butter Dark Chocolate + Protein, and Dark Chocolate Cherry Cashew.

The agency says the bars have too much saturated fat to be labeled "healthy" and not enough protein to use the plus symbol.

Kind's Almond & Coconut bar contains 5 grams of saturated fat per serving, for example, and the Almond & Apricot bar contains 3.5 grams. The FDA requires that healthy snacks contain no more than 1 gram of saturated fat per serving.

The agency says Kind must remove labels including "anti-oxidant rich," "good source of fiber," and "no trans fats" from its products.

"You should take prompt action to correct the violations," William A. Correll, the director of the Center For Food Safety, writes in the letter. He said the FDA will remove Kind bars from stores if the snack company doesn't adjust its labeling.

Kind responded to the charges in a post to its website on Tuesday:

"The FDA is requesting adjustments to the labeling language on four of our bars and our website, and we're working now to bring all items to compliance," the company said. "Please know that there are no quality or safety issues relating to our snack foods or their ingredients. Kind snacks remain a safe and nutritious choice for our fans and their families."

The company said the nut content of its bars is partly to blame for the violations.

"Nuts, key ingredients in many of our snacks and one of the things that make fans love our bars, contain nutritious fats that exceed the amount allowed under the FDA's standard," the company said. "This is similar to other foods that do not meet the standard for use of the term healthy, but are generally considered to be good for you like avocados, salmon and eggs."

Kind is the fastest-growing energy and nutrition bar in the US, according to Fortune. The company's sales have soared from $15 million in 2008 to $120 million in 2012.